1. Field of the Invention
This invention relates generally to lubricating oil compositions and, miore particularly, to aliphatic or aromatic polycarboxylic acid esters, as defined herein, useful as a base oil alone or in admixture with completely synthetic, partially synthetic, or mineral oil based lubricant compositions.
2. Description of the Relevant Art
Modern lubricant compositions, especially lightweight motor oils, are not composed solely of mineral oil components, but include synthetic components. Particularly, lubricating oils for motor vehicles are faced with increasing demands, since the number of revolutions, working pressures and power output are constantly increasing while at the same time the requirement is for high service life and reliability in the engine.
Heavy-duty oils, to which additives are added for a supplementary function such as aging and corrosion protection, high pressure resistance as well as "a dirt carrier," follow wide range oils with flat viscosity curves and suitability for both summer and winter operation. These heavy-duty oils have an extended oil changing interval and even offer decreased fuel consumption, especially in winter operation and in short distance driving. A flat viscosity curve signifies a decreased dependence of viscosity on temperature of the lubricating oil. A measure for the temperature dependence is the viscosity index (VI).
Using a variety of different esters in synthetic lubricating additives is well known in the state of the art. U.S. Pat. No. 4,130,494, for example, discloses certain amine salts of phosphate esters as additives in synthetic lubricating oil formulations. These additives are said to decrease undesired depositing in motors, especially in turbine engines.
U.S. Pat. No. 4,155,861 discloses a lubricant comprising a mixture of a monomeric ester of a branched dicarboxylic acid with an aliphatic primary monoalcohol and a complex ester of a dicarboxylic acid and hexanediol or trimethyl hexanediol. The monomeric diester is always a mixed diester (column 6, lines 1 and 2), and specifically cited in trimethyladipic acid octyl decyl ester. The lubricant combination described is claimed to be characterized by having a universal application.
The known polycarboxylic acid esters are manufactured, for example, on a large industrial scale while using oxo-alcohols as esterification components. Oxo-alcohols, which are especially suited for esters as synthetic lubricants, are manufactured from oligo olefins.
Table I illustrates common oligo olefins and the alcohols able to be prepared from them through oxo-reactions.
TABLE I ______________________________________ Oligo Olefin Oxo-Alcohol ______________________________________ Diisobutene Iso-nonanol Triisobutene Iso-tridecanol Propene dimer Iso-heptanol (Isohexene) Propene trimer Iso-decanol Propene tetramer Iso-tridecanol ______________________________________
From these commercially available oxo-alcohols, polycarboxylic acid esters are prepared, which find application as synthetic lubricants. See, for example, R. C. Gunderson and A. W. Hunt, Synthetic Lubricants, Reinhold Publishing Company, 1962, page 151 and following; and D. Klamann, Lubricants and Related Products, Verlag Chemie, 1984, which are incorporated herein by reference.
An objective of the present invention is polycarboxylic acid esters for use as a base oil alone or in admixture with completely synthetic, partially synthetic, or mineral oil based lubricant compositions which demonstrate an improved temperature/viscosity behavior as expressed by a higher viscosity index and improved low temperature properties and having a lower evaporation loss and higher flash point than known polycarboxylic acid esters.
This objective is surprisingly met by the aliphatic or aromatic polycarboxylic acid esters described in greater detail below.